The present invention pertains to an electronic camera, particularly, to an electronic camera of a type employing an optical low-pass filter and a focal plane shutter. More specifically, the invention pertains to such an electronic camera having an improved mounting arrangement for the optical low-pass filter.
In an electronic camera, an image-bearing optical signal is focused by an objective lens onto an image pickup device which converts the optical signal into an electrical signal which can be stored on a storage medium such as a magnetic tape, magnetic disk or semiconductor memory. This type of camera is advantageous over an ordinary camera which requires the use of film since no chemical processing of any type is required.
The image pickup device used to convert the optical signal into an electrical signal is an array of individual pixels (picture elements). Two primary types of such pickup devices are known, the MOS type and the CCD type. In either case, there is unavoidably some small gap between adjacent ones of the pixels. Hence, if the image being viewed contains spatial frequencies higher than the density of pixels (the number of pixels per unit distance), moire pattern interference results. In order to eliminate such interference, it has been the practice to provide an optical low-pass filter in the path of the image bearing optical signal to eliminate the unwanted high spatial frequency component from the optical signal. A crystal plate and a phase filter are suitable for this purpose.
To control the amount of exposure in an electronic camera, it has been the usual case to employ a focal plane shutter of the same type typically used in film-type single lens reflex cameras. In such a case, a problem arises regarding the placement of the optical filter. Two arrangements have been previously employed: one where the optical filter is positioned between the rear of the objective lens and the image reflecting mirror, and the other where the optical filter is positioned between the focal plane shutter and the image pickup device. However, the former arrangement is disadvantageous in that it is necessary to provide space for the image reflecting mirror to move out of the optical path when a photographing operation is to be carried out. This necessitates moving the objective lens forwardly, hence forcing an increase in the distance between the rear flange of the objective lens and the image pickup device, thereby making the camera bulky as a whole. The latter arrangement is disadvantageous in that the distance between the focal plane shutter and the image pickup device is increased beyond what is a desirably allowable maximum, reducing the efficiency of the shutter and again increasing the distance between the rear flange of the objective lens and the image pickup device, thus still resulting in an overly bulky camera.
The term shutter efficiency will be explained with reference to FIGS. 1 and 2. The shutter efficiency .eta. is defined as: EQU .eta.=T/t=w/[w+(f/D)S],
where S is the distance between the focal plane shutter 1 and the surface 2 of the image pickup element, T (=w/v) is an effective exposure time, t (=[w+(f/D)S]/v) is a total exposure time, w is the slit width of the focal plane shutter, v is the running speed of the focal plane shutter, D is the diameter of the diaphragm of the objective lens 11, and f is the focal length of the object lens 11.